1. Field of the Invention
The present invention relates to a failure diagnostic apparatus for manifold pressure sensors used in an engine control system and, more specifically, to a failure diagnostic method and apparatus for manifold pressure sensors in a system having a first manifold pressure sensor and a second manifold pressure sensor that backs up the first manifold pressure sensor.
2. Description of Background Art
A number of sensors have been used in engine control and failure diagnostic systems for diagnosing whether or not these sensors are normal. For example, Japanese Patent Application Laid-Open No. 2003-307152 discloses a method that includes a step of determining whether or not an output signal of a sensor that senses pressure in an intake manifold is within a preset range, a step of calculating the pressure in the intake manifold on the basis of engine operating conditions, and a step of determining whether or not a difference between the pressure in the intake manifold indicated by the output signal from the sensor and the calculated pressure in the intake manifold is smaller than a set value when the output signal from the sensor is within the range. However, when the output signal from the sensor is not within the range or when the difference is not smaller than the set value, the calculated pressure is set as the pressure in the intake manifold.
In an electronically controlled throttle apparatus disclosed in Japanese Patent Application Laid-Open No. 10-176582, two systems each include an accelerator sensor, a throttle sensor, etc. are provided as a failsafe device or the like. Failure diagnosis is performed by comparing sensor outputs of the two systems. For example, it is determined whether or not the throttle sensor has failed, by comparing a deviation between a main throttle sensor and a sub throttle sensor and a predetermined threshold value.
Failures of a sensor system such as a disconnection and/or a short circuit can be determined when a sensor output value is too high or too low with respect to a predetermined value or a predetermined range. The failures can be addressed in a predetermined manner, depending on the type of failure. However, it is possible that a change or deterioration of output characteristics may impair normal control, even though the sensor output value is within a predetermined range. Therefore, it is not possible to determine that a failure has occurred.
The diagnostic method disclosed in Japanese Patent Application Laid-Open No. 10-176582 can detect that either one of the two system sensors has a failure. However, it is difficult to determine which one of those has the failure.